Intravenous infusion of medicinal liquids has traditionally been accomplished using drip regulated, gravity flow systems. However, it is generally recognized that a more precisely regulated flow of drug to the patient can be administered with a pump. Because of their simplicity and ease of use, peristaltic pumps are often used for this purpose. A gravity flow system is readily converted to a peristaltic pump infusion system by threading the drug delivery or intravenous (IV) line, which is attached to the drug container, through the peristaltic pump. The pump then controls the rate at which the drug is delivered to the patient.
A peristaltic pump displaces liquid by repetitively compressing a section of the flexible tube comprising the IV line. This line is threaded through a channel formed in the pump and extends unbroken, from the drug container to the patient. In one type of peristaltic pump, the channel is curved around a central axis. A rotating arm with rollers fixed on each end compresses the section of tubing disposed within the channel, advancing the rollers along the longitudinal axis of the tubing as the arm rotates about the central axis. Liquid within the tubing is thus forced ahead of the advancing roller along the internal passage in the line.
Another type of peristaltic pump has a linear channel in which the IV line is threaded and is thus referred to as a linear (or traveling wave) peristaltic pump. The linear peristaltic pump includes a plurality of finger-like plungers that are sequentially actuated by cams mounted along a motor driven shaft. Liquid within the section of tubing that extends along the linear channel is advanced along the tubing's longitudinal axis by the advancing wave-like compression of the fingers. An example of such a pump is disclosed in U.S. Pat. No. 4,479,797.
Inlet and outlet valves and a single liquid displacement plunger are used in another type of peristaltic pump. Each pumping cycle in this type of pump begins with the outlet valve closed and the inlet valve open. Fluid flows from the source container into a short section of tubing that is disposed between the inlet and outlet valve. After this section of tubing has filled with liquid, the inlet valve closes and the outlet valve opens. The plunger then compresses the short section of tubing between the valves, displacing the liquid contained therein, and forcing it from the pump. U.S. Pat. No. 4,559,038 discloses a peristaltic pump of this type.
Cassette pumps are also frequently used in administering medicinal fluids to a patient and normally provide a more accurate rate of fluid flow than a peristaltic pump. In a cassette pump, a cassette comprising a plastic housing that includes a pumping chamber and inlet and outlet valves, is connected via a disposable tube set to a drug container. The cassette is inserted into an appropriate device designed to drive the cassette and administer fluid at a controlled rate. The pumping device includes an inlet valve actuator, an outlet valve actuator, and a pumping plunger. Inside the cassette, passages connect the inlet valve and the outlet valve to the pumping chamber; a flexible membrane, which is sealed between two halves of the plastic housing, interrupts fluid flow through inlet and outlet valve openings formed in the housing when the membrane is deformed by the inlet and outlet valve actuators. The plunger acts on the flexible membrane covering the pumping chamber in the cassette to force liquid past the open outlet valve and through an outlet port of the cassette. An example of a cassette pump is disclosed in commonly assigned U.S. Pat. No. 4,818,186.
The rate at which fluid is delivered by each type of positive displacement pump discussed above is normally controlled by the rate at which the pump operates, e.g., the rotational rate of the rotating arm in that type of peristaltic pump. Furthermore, the accuracy with which a given rate or volume of fluid flow can be achieved by these pumps is dependent upon the pressure of the fluid at the input of the pump and the back pressure at its output. Since both the flexible tubing (in the peristaltic pumps) and the flexible membrane (in the cassette) define a compliant pumping chamber, the volume of fluid that fills the pumping chamber is affected by the head pressure of the fluid from the drug container. Similarly, the volume of fluid delivered at the output of the pump depends on the back pressure of the fluid downstream of the outlet. The cassette pump and the single plunger type of peristaltic pump, both of which have positive closure inlet and outlet valves, are particularly sensitive to head and back pressures because the volume of the pumping chamber disposed between the valves and the amount of fluid that fills the chamber generally must be constant to provide an accurate and consistent rate of flow from the pump.
Several other parameters can affect the accuracy of fluid flow delivered by specific types of positive displacement pumps. For example, when the compression force is removed from the tubing in a peristaltic pump, the tubing must recover to a defined and consistent internal diameter to insure that the same volume of fluid is delivered in each pump cycle. If the volume of the passage defining the pumping chamber changes over time, for example, due to changes in the tubing elasticity, the pump's flow rate will also change. Inexpensive polyvinyl chloride (PVC) tubing, commonly used for disposable tube sets in medical IV applications, is known to experience changes in elasticity over time and with repetitive compression of the tubing, thereby affecting the extent to which the tubing recovers when a compression force is removed.
In the single plunger type of peristaltic pump, the plunger should compress the tubing uniformly and consistently with each pumping stroke to provide an accurate and consistent rate of fluid flow from the pump. The plunger mounting assembly must permit the plunger to move freely back and forth along a reciprocation axis, yet should prevent it from twisting or moving laterally away from this axis, because such movement can change the compression stroke volumetric displacement. Since the plunger is typically driven by a rotating cam, the mounting assembly should also provide a biasing force to maintain the plunger in contact with the cam surface, preferably without introducing sliding friction or using helical springs. Most prior art plunger mounting assemblies do not address all of these concerns.
Due to the potential safety concerns involved in administering medicinal fluids intravenously to a patient, an infusion pump should include an air-in-line sensor to detect large air bubbles within the pump and stop the pump before such bubbles are infused into the patient's circulatory system. Provision should also be made to detect when a drug container becomes empty or a supply line connected to the container occluded. If the flow of fluid from the pump is interrupted for any reason, the pump should be shut off and an alarm sounded to alert medical personnel. Ideally, these functions should be integrated into the pump, and are in some prior art pumps. However, virtually none of the available peristaltic pumps currently provide both of these safety-related features.
In consideration of these problems that exist with the prior art pumps, it is an object of the present invention to provide a positive displacement pump in which the volume and rate of delivery of fluid from the pump is substantially unaffected by variations in the pressure of fluid supplied to the pump. Another object of this invention is to provide a pump in which the volume and the rate fluid is delivered from the pump is substantially unaffected by variations in fluid pressure downstream of the pump. Yet a further object is to provide a positive displacement fluid pump that delivers fluid to an output port of the pump at a predefined pressure. Still a further object of the invention is to provide a spring-biased support for a plunger enabling it to reciprocate freely along a reciprocation axis without sliding friction, while preventing it from twisting or moving laterally away from the reciprocation axis. These and other objects and advantages of the present invention will be apparent from the attached drawings and the Description of the Preferred Embodiments that follows.